Accumulators are rechargeable charge storage devices which in a charged state are capable of providing electric power to a load. The voltage at which the electric power is provided is dependent on the type of accumulator. For lithium-ion accumulators this voltage is in the range of 3.3 V. For supplying loads that require higher supply voltages several accumulators may be connected in series, thereby forming an accumulator arrangement. The supply voltage provided by such accumulator arrangement corresponds to the sum of the individual supply voltages of the accumulators connected in series. For charging the accumulator arrangement it is known to provide a charging current to the accumulator arrangement via connection terminals, to monitor the voltage across the individual accumulators during the charging process, and to finish the charging process when the voltage across one of the accumulators exceeds a given threshold. Exceeding this threshold indicates that the corresponding accumulator is completely charged and should not be charged any further.
To optimize the useful capacity of the accumulator arrangement it is known to balance the charge states of the individual accumulators at the end of the charging process. To this end, a fly back converter may be provided that comprises a primary winding connected to one of the connection terminals of the accumulator arrangement and several secondary windings connected to the accumulators. For balancing the charge states in a first phase of operation, energy is taken from the accumulator arrangement, i.e. the series circuit of the individual accumulators, with the energy being stored in the primary winding. In a second phase of operation, the energy stored in the primary winding is transferred to all secondary windings and via the secondary windings is fed to the individual accumulators.
In this arrangement, the highest current flows into that accumulator across which the lowest voltage is present, i.e. which has the lowest charge state. Due to an internal resistance of each accumulator, the voltage across each individual accumulator increases with a current flowing into the accumulator. This voltage increase counteracts the charging current. Accumulators having a low charge state are therefore charged less than it was required according to their charge state. Furthermore, a part of the overall energy taken from the accumulator arrangement is also fed to those accumulators which have already reached a high charge state, or that have been completely charged, respectively.